1. Field of the Invention
The present invention relates generally to an improved method and apparatus for correcting distortions in the information content of multi-channel reproducing systems including, without limitation, video systems (either optical or electronic) and audio systems.
2. Description of the Prior Art
The prior art most relevant to the present invention has arisen in the area of methods and devices for evaluating color photographic negatives and determining color compensation filter combinations to optimize the reproduction fidelity of the print. Representative of this prior art is U.S. Pat. No. 3,761,183 which issued to Yuasa, et.al. on Sept. 25, 1973, entitled Device for Measuring Color-Compensation Quantity for Color Printing. That patent is directed to a system for determining the optimal color compensating filters to be used in a photo enlarger when printing a given negative. The system uses photodetectors sensitive in the red, green and blue spectrum ranges to sense the average spectral content of a given negative. Signals from the respective photodetectors are compared to determine color compensating quantities in the green and blue and the red signal is used to determine exposure time. Operation of the system is premised on the assumption that the overall average color of the negative is neutral gray. Implicitly each color is weighted in the average by the area it occupies. In practical application, however, any given color negative will probably not exhibit an overall neutral gray. The system is therefore subject to error when non-average conditions are encountered. The error is analogous to that introduced by an exposure meter when the subject is backlighted.
U.S. Pat. No. 4,217,648 entitled Method and Apparatus for Evaluating Color Photographic Negatives Prior to Copying which issued to Thurm, et.al. on Aug. 12, 1980, is directed to a system which provides a measure of compensation for negatives exhibiting areas of dominant color. The color density in each of the primary colors in each of a plurality of unit areas of a negative is measured and compared in a computer with the respective densities in adjacent unit areas. The difference between the results of the measurements of two adjacent unit areas is stored only when it is below a predetermined value. The stored results indicate the presence or absence of areas of dominant color and are used to adjust the copying light accordingly. This system represents an improvement over simple averaging in that the more selective sampling permits adjustments of the copying light such that the selected average, now not weighted by area, is corrected to neutral gray. While probably applicable to a broader variety of picture material, the premise that the unweighted average by color area is gray fails to apply universally. For example, in a full face portrait, the flesh, hair and eye colors rarely average to gray. Adjustments of subtractive filters, in the primary colors is in effect an optical adjustment of the gain of each primary color channel. In actual practice, the adjustment of color filters affects not only the gain of each channel but also inter-channel cross-talk over which the Thurm, et.al. system has no explicit control.
The Applicants have found that filters that appear the same color to the human eye can affect the inter-channel cross-talk differently depending upon the details of their spectra in relation to the details of the print material spectra. The Wratten color compensating filters available from Eastman Kodak Co. and used in color photo enlargers are based upon standardized dyes. The dichroic color filters use evaporated films on glass and interference effects to provide the three primary color pass band type spectra with sharp cut offs. Each Wratten filter is calibrated with a number such as CC20Y which indicates a color compensating filter of an optical density of 0.20 in the yellow. If one selects a correction of CC20Y with a dichroic color head the corrective effect will be generally different from that achieved by use of the single Wratten filter. The difference stems significantly from the inter-channel cross-talk effect created by the combination of the separate dichroic color filters.
The prior art systems for correcting color rendition thus are seen to operate on three independent variables; i.e. the primary color channel gains (intra-channel gains). In order to provide any significant improvement in the art it is necessary to also control the inter-channel gains or cross-talks which, for a three channel system requires the coordinated adjustment of at least nine quantities, three of which are intra-channel gains and six are inter-channel gains. When applied to systems for correcting distortions in reproducing systems generally, the number of channels, N, can increase sharply, leading to at least N.sup.2, gains, of which N are intra-channel gains and N.sup.2 -N are inter-channel gains.